An introduction to landscape connectivity

Posted by Zach Carter

Landscape connectivity (also known as ecological connectivity or landscape permeability) is the degree to which a landscape facilitates or impedes wildlife movement. Understanding landscape connectivity has become a major conservation priority for ecological managers because it can be used to protect and restore important ecological processes; such examples include: the promotion of gene flow/dispersal, creation of risk assessments to characterise the likelihood of invasive species dispersal, and quantification of habitat fragmentation throughout peri-urban regions.

Conservation CorridorAn example of landscape connectivity in practice: a conservation corridor that traverses the Trans-Canada Highway in Banff National Park to facilitate large mammal movement (image: https://conservationcorridor.org/2012/10/banff-national-park/)

Most commonly, spatially explicit connectivity models use resistance surfaces to represent landscape features. This is a graph-theoretic technique that reflects movement, represented as a pixel value in a grid within a geographic information system. Connectivity models ultimately use resistance surfaces to calculate the ecological cost associated with movement through a landscape between two termini (starting/ending points). It is assumed that the organism of interest will travel in such a way so as to minimise incurred costs. Accurate representation of these movements can then be used to make informed management decisions based on desired outcomes.

Two common models used for calculating ecological distance include the cost distance and current flow methodologies. These methods propose antithetic assumptions regarding the organism(s) emigration trajectory, where the cost distance model assumes the organism has perfect knowledge of the landscape and will, therefore, choose a path that minimises cumulative ecological costs, and the current flow model which treats the landscape as an electrical circuit and assumes the organism has no prior knowledge of the landscape whatsoever. Often these methods cannot elucidate an organism’s true understanding of the landscape and, as such, are used in conjunction to create a more complete picture.

Resistance Surface ExamplesAn example cost distance (fig. A, least-cost path) and current flow (fig. B, probabilistic movement) output for a generalised mammalian disperser as it emigrates from the New Zealand mainland to an offshore island in Fiordland. The red coloured least-cost path (fig. A) represents the path of least resistance as the organism emigrates. The dark coloured areas (fig. B) represent areas of probabilistic movement from an emigrating organism (source: Z Carter).

Current flow has gained much attention recently for use in connectivity modelling because it considers probabilistic movement across all possible paths within a landscape. If maximising connectivity is the desired ecological outcome (e.g. reducing habitat fragmentation), then calculating current flow between two termini within a landscape is a good model to follow (see fig. B above). On the other hand, if the desired ecological outcome is to reduce organism dispersal (e.g. prevent the spread of invasive species) calculating the least-cost path between termini may be a good modelling option because it often overestimates connectivity. In this instance it would be better to overestimate connectivity than to under estimate it in order to produce informative ecological recommendations regarding the potential spread of a pest species.

For more reading I recommend the following publications:

Etherington, T. R. (2015). “Geographical isolation and invasion ecology.” Progress in Physical Geography 39(6): 697-710.

McRae, B. H. and P. Beier (2007). “Circuit theory predicts gene flow in plant and animal populations.” Proceedings of the National Academy of Sciences 104(50): 19885-19890.

Wade, A. A., et al. (2015). “Resistance-surface-based wildlife conservation connectivity modeling: Summary of efforts in the United States and guide for practitioners.” Gen. Tech. Rep. RMRS-GTR-333. Fort Collins, CO: US Department of Agriculture, Forest Service, Rocky Mountain Research Station. 93 p. 333.

 

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Zach Carter is PhD candidate in the School of Biological Sciences at the University of Auckland. His research focuses on developing prioritisation models to assist eradication efforts for the Predator Free 2050 Programme. He is supervised by James Russell and George Perry.

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The allee effect: an effective ally to achieve eradication of invasive species?

Posted by Hester Williams @HesterW123

The rise in biological invasion, strongly related to increasing international trade and travel, is creating global ecological and economical challenges.

The process by which biological invasions occur can be divided into three phases: arrival, establishment, and spread. Early intervention in the form of detection and eradication can be one of the most cost-efficient approaches. Eradication is the deliberate elimination of an invading species from an area, and is greatly assisted by prompt detection when the newly established population is still small and not widely spread.

Given the perceived difficulty of eliminating all individuals of a species, the practicality of eradication has often been questioned. However, recent population studies indicate that low density populations of a variety of species are governed by Allee effects and this may facilitate eradication. Allee effects may arise from a variety of mechanisms (e.g. mate-location failure, failure to overcome host defences, failure to satiate predators) and create a population threshold, below which population growth rate is negative. Consequently, eradication may not require directly eliminating all individuals in a population; instead, it may only be necessary to reduce the population below the Allee threshold, and extinction will proceed without further intervention.

The loss of habitat and fragmentation, which are detrimental to rare and endangered species, are complementary in attempts to eradicate an invasive species from an area. Although habitat loss is not a cause of an Allee effect, it can reduce population size such that the population could then become succeptible to an Allee effect. Fragmentation of an invasive species population (through management actions such as host removal /fragmentation) could result in reduced patch-to-patch dispersal as well as reducing the population densities in each fragmented patch to below the Allee threshold. Thus, sufficiently small and distant patches could lead to extinction of the population.

My studies use Neolema ogloblini, a biocontrol agent for Tradescantia fluminensis, as proxy for an invasive insect pest species (Fig 1).

Fig1

Fig 1: The leaf beetle, Neolema ogloblini, a biocontrol agent for Tradescantia fluminensis, with typical adult damage.

Experiments completed last summer have indicated that at small population sizes, establishment of this beetle is moderated by an Allee effect. This summer I will test the effectiveness of host removal as a management tool to achieve eradication by exploiting the Allee effect. I will remove a selected number of host patches within a meta-population of Neolema ogloblini, thereby fragmenting the remaining population and in turn subjecting it to Allee effects to achieve eradication (Fig 2).

Fig2

Fig 2: Host removal as management tool to achieve eradication through exploitation of the Allee effect. A selected number of host patches within a meta-population of Neolema ogloblini will be removed (denoted by white patches), fragmenting the remaining population and in turn subjecting it to Allee effects to achieve eradication.

Results of this experiment will ultimately give guidance on what eradication approaches are more or less promising for particular invasive species.

 

Hester Williams is a PhD candidate in the School of Biological Sciences, University of Auckland and is stationed with the Landcare Research Biocontrol team in Lincoln, Canterbury. She is interested in invasion processes of both insect and plant species. Hester is supervised by Darren Ward (Landcare Research/University of Auckland) and Eckehard Brockerhoff (Scion), with Mandy Barron (Landcare Research) as an advisor. Her studies are supported by a joint Ministry for Primary Industries – University of Auckland scholarship. The project is an integral part of an MBIE program “A Toolkit for the Urban Battlefield” led by Scion.